Protein A is a cell surface protein found in Staphylococcus aureus. It has the property of binding the Fc region of mammalian antibodies of class IgG, but the affinity varies with host species and antibody subclass. (For a comprehensive review, see Langone, J. J. [1982] Advances in Immunology 32:157-252.) Protein A can be isolated directly from Staphylococcus cell walls, or from the growth media of mutant strains that secrete protein A. In addition, the gene for protein A has been cloned and expressed in Escherichia coli (Lofdahl, S., Guss, B., Uhlen, M., Philipson, L. and Lindberg, M. [1983] Proc. Natl. Acad. Sci. USA 80:697-701; Colbert, D., Anilionis, A., Gelep, P., Farley, J. and Breyer, R. [1984] J. Biological Response Modifiers 3:255-259). This has allowed the production of large amounts of recombinant protein A.
Several applications based on the IgG binding properties of protein A have been developed. These include the following: